1. Field of the Invention
The present invention relates in general to a method of producing a polyamide resin. More particularly, the invention is concerned with a method of producing a heat-resistant polyamide resin which has a high molecular weight and which can be molded in a molten state with high stability.
2. Related Art
Nylon 66, 46, 6T and an aramide resin have been mass-produced as a heat-resistant polyamide resin which exhibits excellent mechanical, chemical and physical properties. In general, these nylon 66, 46, 6T and etc. are produced by dehydration and polycondensation of a nylon salt which is obtained by reaction of an organic dicarboxylic acid and an organic diamine. However, this method requires cumbersome process steps, and the polycondensation needs to be effected at a high temperature for a long period of time. In producing the aramide resin, there are some problems: use of expensive organic dicarboxylic dihalide; corrosion of the reaction device due to halogen; difficulty in recycling the material; and contamination of the environment.
The inventor of the present invention proposed in U.S. patent application Ser. No. 08/825,500 filed Mar. 31, 1997, now U.S. Pat. No. 5,837,803 a novel method of producing a polyamide resin by using a polyester resin. This method is totally different from the above-indicated conventional method of producing the polyamide resin by polycondensation, and is free from the problem of corrosion of the reaction device. Explained in detail, according to the proposed method, the conventionally used organic dicarboxylic acid is replaced with the polyester resin. The polyester resin is reacted with, in a predetermined solvent as a reaction medium, a diamine compound whose molar ratio is determined to be in a specified range for the polyester resin. According to the proposed method, the polyamide resin can be obtained in a simplified manner in a relatively short period of time.
For increasing the molecular weight of the polyamide resin obtained according to the proposed method to a sufficiently high extent suitable for practical use, the polyamide resin needs to be subjected to polycondensation in a solid state (hereinafter referred to as "solid-polycondensation) or polycondensation in a molten state (hereinafter referred to as "melt-polycondensation"). Alternatively, it is necessary to increase the reaction temperature and reaction time in an attempt to increase the molecular weight of the polyamide resin. However, the additional step of the solid- or melt-polycondensation undesirably pushes up the cost of manufacture of the polyamide resin, while an increase of the reaction temperature or time causes undesirable problems that the reaction product tends to form an aggregate and adheres to the walls of the reactor, thereby deteriorating the production efficiency of the polyamide resin.
According to the proposed method described above, the polyethylene terephthalate resin (hereinafter referred to as a "PET" resin) is formed into a polyamide resin having a considerably high melting point, such as a polyethylene terephthalamide (whose melting point is 455.degree. C.), a polyphenylene terephthalamide (whose melting point is 420.degree. C.), or a polyhexamethylene terephthalamide (whose melting point is 350.degree. C.).
These polyamide resins, however, had problems when they are molded in a molten state. Since these polyamide resins have considerably high melting points, they cannot be suitably melted if the molding temperature is relatively low. On the other hand, if the molding temperature is excessively high, the polyamide resins suffer from thermal degradation. Thus, the polyamide resins obtained according to the proposed method can not be molded with high stability.